The present invention relates to improvements in a power steering apparatus for an electric vehicle, and more particularly to a hydraulic pressure type power steering apparatus where a steering assisting-power is generated by the fluid pressure of the fluid pump driven by an electric motor.
A variety of power steering apparatuses for electric vehicles have been proposed. A Japanese Patent Provisional Publication No. 5-270424 discloses a typical power steering apparatus for an electric vehicle in which a hydraulic pump driven by an electric motor generates assist-power for steering. A steering mechanism, which steers a pair of steered wheels in the steered direction according to a rotational degree of a steering wheel, is assisted by a piston mechanism moved by the hydraulic pressure supplied from the electric hydraulic pump. Simultaneously, a first setting means sets a first assist power according to a vehicle speed and a steered condition during a steering period, and a second setting means sets the electric hydraulic pump to a stop mode in an extremely low speed range and sets the electric hydraulic pump into a minimum operation mode in a relatively high speed range as compared with the extremely low speed range during a non-steering period. This conventional apparatus improves the energy consumption of the hydraulic pump in the non-steered condition in the extremely low range, and improves the starting-up ability in a range higher than the extremely low speed range by setting the hydraulic pump in a minimum operation mode.
However, this conventional apparatus yet has a problem that the steer responsiveness is degraded by the deviation of the electric source voltage.